Animal tissue that is often used in implantable bioprosthetic devices or to repair damaged tissue in a patient is typically provided to medical personnel stored with or previously treated with chemical agents such as formaldehyde and/or glutaraldehyde. This chemical treatment helps to prevent rejection of the implanted bioprosthetic device by the recipient, provides for sterilization and helps to stabilize the proteins in the tissue, thereby making the tissue or device containing such tissue more resistant to mechanical fatigue and degradation by proteolytic enzymes. Formaldehyde and glutaraldehyde, for example, stabilize the tissue by forming cross-links with collagen and other proteins in the tissue. Also, the tissue is often stored in a dilute aqueous solution containing glutaraldehyde (and/or formaldehyde) to maintain the tissue components in a hydrated state and to maintain a microbe-free environment. However, tissue containing these agents must be extensively rinsed prior to surgical implantation. Moreover, an important consideration in selecting a biological tissue for implantation is that the tissue should be readily available to medical personnel in as close to a ready-to-use form as possible, that is, with minimal preparation prior to surgery. This reduces the opportunities for error and also the implantation time.
Attempts have been made to develop procedures to pre-treat biological tissue and bioprosthetic devices containing biological tissue so that such tissue and devices can be provided to medical personnel substantially free of formaldehyde or glutaraldehyde, and to provide the biological tissue in a substantially dry form that is ready for implantation or rehydration. One attempt to address this issue involved immersing the biological tissue in aqueous C1-C3 alcohol, for example aqueous ethanol solutions, see, U.S. Pat. Nos. 6,277,555 and 6,630,001 to Duran et al. The solution treated tissue is then immersed in an aqueous glycerol solution or low molecular weight (<1000 D) polyethylene glycol. Thereafter, the tissue is briefly immersed in aqueous heparin solution, frozen and lyophilized.
U.S. Pat. No. 4,300,243 describes the preparation of biological collagenous tissue for surgical implantation. The preparation of the tissue includes contacting the tissue with fresh amounts of a water-miscible organic solvent selected from the group consisting of methanol, ethanol, propanol, isopropanol, acetone, methylethylketone and mixtures thereof. The excess alcohol is then removed at a pressure of between 0.1 bar to 1.0 bar. However, it is known that such dehydration processes significantly reduce the overall dimensions of the tissue, that is, the tissue shrinks. Also, biological tissue subject to this dehydration process cannot be successfully rehydrated and returned to substantially its original dimensions. As a result, bioprosthetic devices comprising tissue components that have undergone such a dehydration process are not good candidates for implantation. See, U.S. Pat. No. 6,534,004, col. 1, lines 53-65.
U.S. Pat. No. 6,534,004 describes a process of providing bioprosthetic devices containing a biological tissue for dry storage. The process includes treating the biological tissue with an aqueous solution of organic compounds having a plurality of hydroxyl groups (e.g., polyhydric alcohols, polyglycols or water soluble carbohydrates). One of the more preferred treatment solutions is an aqueous solution of glycerol or derivatives of glycerol. The biological tissue is immersed in this solution, removed and permitted to dry in air. The tissue is then sterilized and packaged for storage.
All the foregoing prior art approaches to preparing tissue for dry storage require the use of aqueous solutions. However, the use of aqueous solutions makes the tissue susceptible to microbial contamination. It is desirable, therefore, to totally eliminate water from the process and to thus render the dried tissue less prone to microbes. The present invention provides such a process. It provides new methods for preparing biological tissue or bioprosthetic devices containing such tissue suitable for sterile dry storage, and available to medical personnel in as close to ready-to-use form as possible.